Common masonry walls are made of hollow concrete blocks and mortar; the hollow portions of the blocks are typically referred to as “cells”. The cells reduce the weight of block that the mason must lift into place during construction, and also enable vertical reinforcement to be installed in the wall. For added resistance to lateral loads, grout and vertical reinforcements, such as steel reinforcing bars, are placed in the cells of the block. Filling of the block cells also enhances the compression strength of concrete block walls under vertical loads. Placing vertical steel reinforcing bars in the block cells enhances the flexural strength of the wall to improve ductility through yielding of this reinforcing bar. However, the extent of ductility is limited by compression failure of the concrete block at relatively low compression strain.
In seismic design for earthquake loading, concrete block shear walls that are intended to resist the horizontal forces caused by seismic motion must be reinforced to increase their flexural strength and to develop some ductility and energy dissipation properties. However, it is very challenging to achieve sufficient ductility and energy dissipation prior to compression failure of the concrete block. Reinforced concrete block construction must often be designed for nearly twice as much lateral loading as the more ductile competing construction materials such as reinforced concrete structures and steel structures. Hence, reinforced concrete block construction is often not economically competitive and sometimes not technically feasible. Changes in recent building codes have imposed limitations affecting reinforced masonry construction with the result that use of this most common building material has been significantly limited.
Another aspect of structural design relates to the limit states with which building design must comply, namely the “serviceability limit state” and the “ultimate limit state”. The serviceability limit state deals with the normal course of building performance under expected loads, and requires that in these circumstances the building should not show any sign of distress and should function in the intended manner. The ultimate limit state is directed to providing a margin of safety against failure by designing for a higher load than is actually anticipated and by making allowance for variability in material strength, for example to deal with unexpected overloading or weaknesses that may develop.
More recently, the concept of design to accept damage but prevent collapse has been introduced, particularly in relation to seismic forces and other forces that are more difficult to predict. This design concept is directed to conditions beyond the ultimate limit state, at which point permanent damage is experienced. Where a structure has been appropriately designed to accept damage but prevent collapse, in conditions beyond the ultimate limit state but within the design limit, the structure may be visibly damaged but will retain most (at least 80%) of its original strength and, in the case of earthquakes, the additional accepted damage produces increased ductility and energy dissipation. This additional ductility and energy dissipation allows design for lower lateral forces for cases of low probability of occurrence such as either the 1 in 475 year or 1 in 2500 year earthquake that is currently designed for in most countries. In the event of such an earthquake, damage would occur but the building would not collapse and thus deaths, injuries and collateral damage may be reduced. Depending on the extent of damage, it might be economical to repair the building.
There are two related but separate aspects of behavior of grout-filled hollow masonry block construction subjected to vertical compression, such as is created in concrete block shear walls by gravity loading and by loading resulted from lateral seismic forces: interaction between grout in the cells and the mortared hollow masonry block, and brittle compression failure of grout-filled hollow masonry block.
Reference is first made to the interaction between grout in the cells and the mortared hollow masonry block. In standard hollow block construction, compression failure occurs at stresses well below the compressive strength of individual blocks as a result of incompatibility between the mortar and the block material. Under vertical compression, the larger lateral expansion of the softer mortar creates lateral tension in the blocks which results in development of vertical cracks through the webs and face shells of the block, leading to sudden crushing of the combined material at relatively low levels of vertical strain. Thus, compressive strength of the combination can be predicted based on mortar type and compressive strength of the block. However, when grout is used to fill the cells created in the hollow concrete block construction, addition of this third material creates a more complex condition where the different stress-strain properties of the grout, the discontinuities in the column of grout created by imperfect alignment of the block cells from course to course, wedging action due to the tapered shape of face shells and webs, and shrinkage of the grout, all combine to produce a lower material strength than attained in the ungrouted assemblage. The addition of the grout increases the overall capacity of the structure but, when considering the increased solid area of the grouted cross-section, the stress at failure is typically about 25% lower than for ungrouted hollow masonry, with strength based on failure load (capacity) divided by the effective net area of the assemblage. Increasing the grout strength has only a minor effect on the overall compressive capacity.
Although changes in geometry of the cells in the hollow masonry block and use of shrinkage compensating grout can reduce the decrease in observed strength, these approaches are not fully effective and have undesirable economic impact. Reducing the volume of grout to about 25% of the gross volume and improving the vertical alignment of the cells in successive courses of block masonry can help address the undesirable decrease in strength. For example, for a nominal 20 cm (8 inch) block, a 100 mm (4 inch) diameter cylindrical shaped cell occupies approximately 21% of the gross volume and gross cross-sectional area and, combined with positioning of these blocks so that the cells align from course to course, results in higher compressive strength than traditional grouted hollow block construction.
Turning now to brittle compression failure of grout filled hollow masonry block, despite the improved compressive strength created by the block geometry described above, the mode of compression failure remains the same: development of vertical cracks and sudden crushing/crumbling of the grouted assemblage. This brittle property of grouted masonry and of concrete products in general has been understood for some time as a limiting factor in use of concrete block construction, particularly for seismic design where economic design requires ductile behavior.
It has been shown that lateral confinement of brittle materials such as concrete creates a state of tri-axial compression under vertical axial compression loading so that both higher strengths of the material are obtained and much higher vertical strains are reached prior to crushing and crumbling of the block under the vertical compression load. Both the strength increase and the greater deformability can be used to create more ductile reinforced concrete block shear walls to better resist lateral earthquake load.
A number of strategies have been employed in attempts to introduce lateral confinement into grouted concrete block construction. These confining methods are generally passive in that vertical deformation is required to introduce the confining effects. With vertical compression of the material, lateral expansion of the material takes place where the ratio between the amount of lateral expansion and the vertical compression is known as Poisson's Ratio. At low levels of loading, this ratio is about 0.21 but at high levels of stress this can increase significantly and create what is referred to as dilation. Introduction of confining reinforcement to resist the lateral expansion introduces tension in the horizontal (lateral) reinforcement and a balancing amount of lateral compression in the grouted concrete block. The tri-axial state of compressive stress in the confined region is what creates the much higher compressive strength and greatly increased deformability of the confined material.
One method using a confining reinforcement to enhance the compression capacity and deformability of a grouted section involves placing steel wire mesh, perforated plates, and/or fiber reinforced polymer (FRP) fabric/laminates within the mortar bed joints. For example, Priestley (Priestley, M. J. N. Ductility of Unconfined and Confined Concrete Masonry Shear Walls. TMS Journal, July-December 1981, pp. 28-39) studied concrete masonry prisms confined with 3 mm thick stainless steel plates within the mortar beds. The plates were cut to the net shape of the masonry units so that there was no interference with the grouted cells, with a 5 mm edge allowance for pointing the mortar bed joints. The confined prisms showed increased strength, higher strains at peak load, and a much flatter falling branch of the stress-strain curves. PCT Patent Application No. PCT/US2005/25477, published as WO2006/020261, teaches other methods using confining reinforcement.
U.S. Pat. No. 5,809,732 teaches concrete masonry blocks with one or more external plates that are formed with the plates anchored through the block to enable items to be anchored to a wall built with these blocks. A masonry wall can be constructed using masonry blocks with external plates at preselected locations to anchor items to the wall by attaching them to the plates. The external plates are directed to supporting the anchoring function rather than to reinforcing the wall.
Another proposed technique was to provide lateral confinement for just the grout, for instance by using a spiral coil shape of reinforcement placed inside the block cell prior to grouting.
Hart et al. (Hart, G. C. et al. The Use of Confinement Steel to Increase the Ductility in Reinforced Concrete Masonry Shear Walls. TMS Journal, July-December 1988, pp. 19-42) conducted a comprehensive test program to investigate different types of confinement such as wire mesh, a modified “Priestley Plate”, hoops and spirals. In order to maintain consistent vertical reinforcing throughout all prism tests, one No. 6 bar was provided in each cell. The conclusions were: (1) unreinforced and vertically reinforced unconfined prisms behaved identically and failed in a brittle manner; (2) all types of confinement had a positive effect on the descending portion of the stress-strain curve and increased the area under the stress-strain curve; (3) the Priestley Plate provided the greatest confinement; and (4) the open wire mesh confinement type performed well.
For concrete block construction with standard block sizes, placing confining reinforcements within the mortar bed joints, as suggested by Priestly, means using a 200 mm (8 inch) vertical spacing between the confining reinforcements (i.e., the distance between successive bed joints). Such a large spacing limits the effectiveness of the confinement and effectiveness of support against buckling of enclosed vertical compression reinforcement. Reducing the height of the blocks to reduce the spacing distance demands handling more blocks and laying more mortar, which can dramatically increase construction cost. Similarly, increased construction labour is associated with placement of spiral coil reinforcements inside the block cell prior to grouting. In addition, the effectiveness of such reinforcement is limited because, for a typical grouted cell occupying less than 45% of the solid volume, less than 30% of the section can be effectively confined. Following crumbling of the block and grout outside of the spiral, the residual confined area is prone to buckling and cannot develop sufficient extra strength to compensate for the area lost after the material outside of the confined region fails in compression.
Thus, achieving increased ductility in masonry block construction using techniques mentioned above involves practical difficulties and may also involve significantly increased labour costs.